Mapping the soil underneath ground is of great interest in many areas. Before constructing anything above ground, houses, roads and bridges, it is essential to know what is beneath them. Mapping the deep soil characteristics including its compressibility, its conductivity, its moisture content and the presence of voids and air cavities whether natural or man-made, are parts of the general construction requirements.
The general theory of tomography is well known (see for example Principles of Computerized Tomographic Imaging, IEEE Press 1988—by A. C. Kak and Malcolm Stanley). In principle mapping what is inside an object, is nothing more than solving a set of (n) independent equations of (n) unknowns, part of high school algebra. In the case of a substantially round object, for example a cross section of the human body, the number of independent equations may be established by “looking” at the object from (n) different directions around it, where every “view” is slightly different, but in the total all the necessary different combinations may be viewed.
Looking at the Tomography theory from this simplistic point of view, the major problem in “soil tomography” is that it is hard to view an object in the soil or a portion of it, from all the angles around it, in the soil. The second major difference is the “straight-line-rays” technology; while in a medical CT, a collimated X-Ray beam, may be viewed as a “straight-line-ray”, there is no such technology in “soil tomography”. Viewing a focused electromagnetic beam as a “straight-line-ray” is a good approximation only at microwave frequencies, that do not penetrate the soil, but not at meter long wavelengths, that do penetrate it.
The soil tomography literature is replete with papers trying to circumvent these basic limitations; see for example “Giroux et al. in Computers & Geosciences 33 (2007) 126-137”.
It also has to be realized that the attempts to define the soils in general, in terms of their conductivity(σ), permittivity(∈r) and permeability (μ) and their respective functions of frequency, is an approximation of the reality; the field measurements in soils often do not fit theory, but in very gross lines. Consequently soil maps too, based on further approximations needed to accommodate certain mathematical techniques often misrepresent reality of specific cases. Consequently simple models that have a limited validity and assumptions applicable to specific cases are of great practical value.
Compressible soils and air cavities whether natural or man-made, like abandoned mines, may cause over time, instabilities of structures such as roads, bridges and hi-rise buildings built over or near them. Mapping the underground soil before erecting any structures over or near them is essential, before and after construction, as the underground may change in time due to many factors, some natural and some man-made.
In agriculture, soil moisture and its compressibility are major factors dependent on the structure and composition of the deeper layers and consequently need to be mapped ahead of time.
In archeology too, a method for mapping underground air cavities at moderate depths, without damaging the top layers, is of great value, specially under sites previously discovered and partially excavated. Mapping of the deeper layers may point to where to excavate instead of digging on intuition only, consequently saving time and money.
Enforcing border controls and stopping traffic of illegal merchandise is a problem in many countries given the fact that, a large part of the illegal traffic has been driven underground, mainly using existing natural conduits, water tubes, and drains. The policing of these known routes have pushed traffickers to dig and excavate sophisticated new conduits in isolated areas. Consequently a method for prospecting for passageways in the underground soil is of great benefit for policing border lines.